The present invention relates to a radiation image detecting system with the radiation image receiving plane constituted of two-dimentionally arrayed radiation sensors as pixels, and more particularly to such a system improved in its signal-to-noise ratio.
A typical example of the arrayed-sensor type radiation image detecting system has its image recieving plane constituted, as is shown in FIG. 4, with a plurality of highly sensitive radiation sensors D.sub.ij (i=1, 2, . . . , m; j=1, 2, . . . , n) with a radiation receiving area smaller than ne square millimeter. Many such small sensors D.sub.ij are densely arranged so as to form a (m, n) sensor matrix having a necessary total area, for example, of about 30 cm square. Each of the sensors is made of a flat semiconductor piece plated on both surfaces with metallic layers as electrodes. The semiconductor piece is of a compound semiconductor such as CdTe, HgI.sub.2 or the like. With a bias voltage kept supplied to the sensor through the electrodes, a radiation photon incident to the semiconductor part produces electric charges therein and makes the sensor output between the electrodes a charge pulse signal having an intensity proportional to the energy of the photon. According to FIG. 4, all of the upper-side electrodes of n radiation sensors aligned on each of m rows of the (m, n) sensor matrix are commonly connected and led to corresponding one of m row-side amplifiers A.sub.i, while all of the lower-side electrodes of m radiation sensors aligned on each of n columns of the (m, n) sensor matrix are commonly connected and led to corresponding one of m column-side amplifiers B.sub.j through corresponding one of capacitors C.sub.j. The capacitors C.sub.j are to protect the amplifiers B.sub.j from a bias voltage V.sub.b supplied to the radiation sensors from the lower side. With the amplifiers thus provided, a radiation photon incident to a particular radiation sensor D.sub.ij causes a row-side amplifier A.sub.i and a column-side amplifier to output their respective output pulse signals. In other words, a combination of outputs from two particular amplifiers A.sub.i and B.sub.j determines the radiation sensor irradiated by a radiation photon. However, when two or more photons coincidentally irradiate one sensor or different sensors D.sub.ij, D.sub.ij ', D.sub.ij ". . . (or D.sub.ij, D.sub.i '.sub.j, D.sub.i ".sub.j. . . ) whose outputs are to be inputted to a common row-side amplifier A.sub.i (or B.sub.j), namely, when both or either of the amplifiers A.sub.i, B.sub.j outputs a signal larger than that corresponding to one photon, the coincidently irradiated different sensors can not be distinguished from each other or one another. In such a case, therefore, the outputs form the amplifiers A.sub.i and B.sub.j are to be ignored and not employed as pixel signals. Further, since the height of the output pulse signals from the amplifiers A.sub.i and B.sub.j is proportional to the energy of the photon which irradiates the sensor corresponding to the amplifiers, it also is possible, according to this type radiation image detecting system, to selectively obtain a specific radiation image due to a radiation having a particular wave length by discriminating the output signals of the amplifiers with respect to their pulse height.
In the above radiation image receiving system, each of the amplifiers has its input stage constituted with a charge amplification type pre-amplier making use of a field effect transistor. The output noise power of a charge amplification type amplifier is known to increase in proportion to the square value C.sup.2 of the input-side capacitance C of the amplifier. In general the noise predominates in the output with C.gtoreq.10 pF.
Since the capacitance C may well be considered equal to the output-side capacitance of the signal source to the amplifier, and each of the amplifiers A.sub.i, B.sub.j in the present case has its input connected with n or m radiation sensors in parallel, the input-side capacitances of A.sub.i and B.sub.j are respectively nC.sub.d and mC.sub.d, where C.sub.d is the capacitance concerned with one radiation sensor D.sub.ij (and its wiring). Accordingly, the noise powers inputted to the row-side and the column-side amplifiers A.sub.i and B.sub.j are (nC.sub.d) .sup.2 and (mC.sub.d).sup.2, respectively. This means that the noise of the system increases in proportion to the square of the number of radiation sensors constituting pixels, causing the signal-to-noise ratio (in voltage or current) to increase in proportion to the number of sensors concerned. In addition, the long wirings from the sensors to the amplifiers, which have high input impedance, also cause the amplifiers to be liable to pick up external noises.